1. Field of the Invention
The present invention relates generally to lithographic projection apparatus and more particularly to lithographic projection apparatus incorporating purge gas within the optical system.
2. Background of the Related Art
In general, lithographic apparatus such as are described herein include a radiation system for supplying a projection beam of electromagnetic radiation which may have a wavelength of 250 nm or less, for example. Such apparatus further generally include a support structure for supporting patterning structure which patterns the projection beam according to a desired pattern, a substrate table for holding a substrate and a projection system for projecting the patterned beam onto a target portion of the substrate.
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.
To reduce the size of features that can be imaged using a lithographic projection apparatus, it is desirable to reduce the wavelength of the illumination radiation. Ultraviolet wavelengths of less than 200 nm are therefore currently contemplated, for example 193 nm, 157 nm or 126 nm. Also contemplated are extreme ultraviolet (EUV) wavelengths of less than 50 nm, for example 13.5 nm. Suitable sources of UV radiation include Hg lamps and excimer lasers. EUV sources contemplated include laser-produced plasma sources, discharge sources and undulators or wigglers provided around the path of an electron beam in a storage ring or synchrotron.
In the case of EUV radiation, the projection system will generally consist of an array of mirrors, and the mask will be reflective; see, for example, the apparatus discussed in WO 99/57596, incorporated herein by reference.
Apparatus which operate at such low wavelengths are significantly more sensitive to the presence of contaminant particles than those operating at higher wavelengths. Contaminant particles such as hydrocarbon molecules and water vapor may be introduced into the system from external sources, or they may be generated within the lithographic apparatus itself. For example the contaminant particles may include the debris and by-products that are liberated from the substrate, for example by an EUV radiation beam, or molecules produced through evaporation of plastics, adhesives and lubricants used in the apparatus.
These contaminants tend to adsorb to optical components in the system, and cause a loss in transmission of the radiation beam. When using 157 nm radiation, a loss in transmission of about 1% is observed when only one or a few monolayers of contaminant particles form on each optical surface. Such a loss in transmission is unacceptably high. Further, the uniformity requirement on the projection beam intensity for such systems is generally less than 0.2%. Localised contamination on optical components can cause this requirement not to be met.
Previous methods for cleaning optical components include, for example, the use of ozone as a cleaning material. However, ozone is a very unstable material and degrades only a few hours after its formation. If ozone is to be used to clean the optical surfaces, it is therefore necessary to produce it either in situ, or immediately before cleaning. An ozonizer may, for example, be used for this purpose. However, the extra step of producing the ozone itself is highly inconvenient.
It is therefore one aspect of embodiments of the present invention to provide a lithographic projection apparatus in which optical components can be cleaned with stable cleaning materials.
This and other features in accordance with embodiments of the present invention may be incorporated in lithographic apparatus as described above, wherein the apparatus further includes a gas supply for supplying a purge gas to a space in said apparatus, said space containing an optical component, and wherein said purge gas comprises an oxygen-containing species selected from water, nitrogen oxide and oxygen-containing hydrocarbons.
The inventors have found that the cleaning of optical components in a lithographic projection apparatus can be carried out by addition of relatively low partial pressures of stable oxygen-containing speciesxe2x80x94for example water, nitrogen oxides (NOx) or oxygen-containing organic species such as alcoholsxe2x80x94to a purge gas which is fed to spaces through which the projection beam travels. As these materials themselves are not effective as cleaning agents, they are used in combination with UV radiation. The UV radiation cracks the oxygen-containing species to produce atomic oxygen or other oxygen containing radicals, which are highly effective cleaning agents. Amongst the oxygen-containing species, water was found to remove contaminants at a higher rate than molecular oxygen. With the said low concentrations of cleaning agent in the purge gas, the optical components can be cleaned while projecting a mask pattern onto a target portion with acceptable transmission loss due to absorption of UV radiation by the oxygen-containing species.
After cleaning according to the invention, the transmission or reflection of the radiation beam is increased and the uniformity may also be improved. The invention therefore provides a highly effective method of cleaning optical components in lithographic projection apparatus. It avoids the use of unstable materials such as ozone. Above all, it prevents very time-consuming dismounting of optical components (e.g. lens elements) out of the lithographic projection apparatus in order to clean the component in a separate cleaning unit.
However, patterning structure like a mask for example may be easily taken from its support structure. And in case a plurality of masks are stored in a mask storage box, an already-contaminated mask may be taken out of the storage box and may be cleaned before providing it to the support structure. For both scenarios, it is desirable to provide the lithographic projection apparatus with a separate cleaning unit. With such a cleaning unit, a mask can be readily cleaned after which it is ready for transport to the support structure and subsequently for exposure within a relatively short period of time. An alternative is an external cleaning unit, for which the mask to be cleaned must be taken out of the apparatus to the external cleaning unit, cleaned and brought back to the apparatus, which procedure obviously takes much more time compared to cleaning in the said internal cleaning unit. Moreover, the risk of re-contamination is much higher for an external cleaning unit due to the longer period of time between cleaning and exposure.
According to a further aspect of the present invention there is provided a device manufacturing method including providing a substrate that is at least partially covered by a layer of radiation-sensitive material, providing a projection beam of electromagnetic radiation having a wavelength of 250 nm or less, using patterning structure to endow the projection beam with a pattern in its cross-section, projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material, and cleaning an optical component for use in the apparatus by irradiating a space containing said optical component and/or said patterning structure with radiation having a wavelength of less than 250 nm in the presence of an oxygen-containing species selected from water, nitrogen oxide and oxygen-containing hydrocarbons.
In yet a further aspect of the invention a cleaning unit for cleaning contaminated objects is provided including a space, a radiation source for supplying and directing into said space radiation having wavelengths of 250 nm or less and a gas supply for supplying a purge gas into said space, wherein said purge gas includes an oxygen-containing species selected from water, nitrogen oxide and oxygen-containing hydrocarbons.
Such a cleaning unit is capable of cleaning contaminated objects to which hydrocarbon adlayers are adhered. These objects are not limited to the above-mentioned optical components or patterning structure, but can be any object contaminated with hydrocarbon adlayers.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV or XUV) radiation (e.g. having a wavelength in the range 5-20 nm such as 12.5 nm) or soft x-rays, as well as particle beams, such as ion beams or electron beams.